The design of radio-frequency power amplifiers for communications applications often involves a trade-off between linearity and efficiency. Power amplifiers are typically most efficient when operated at or near the saturation point. However, the response of the amplifier at or near the point of saturation is non-linear, resulting in both phase and amplitude distortion (e.g., intermodulation products). The non-linear response of a power amplifier causes out-of-band emissions and reduces the spectral efficiency in a communication system. In most communication systems, constraints are imposed on the linearity of the transmitted signal both inside and outside the operational band.
One way to improve a power amplifier's efficiency and its overall linearity is to predistort the input signal to the power amplifier, in order to compensate for the distortion introduced to the input signal by the power amplifier. In effect, the input signal to the power amplifier is intentionally distorted so that the added distortion, i.e. predistortion, cancels or at least mitigates the distortion introduced by the power amplifier. Generally, the predistortion is applied to the input signal digitally at baseband frequencies, i.e., before the input signal is up-converted to radio frequencies.
The appropriate predistortion is determined by a predistortion model that is dynamically adapted based on a feedback signal from the output of the power amplifier. To the extent that the output of the power amplifier is distorted, the predistortion model can be adjusted based on the feedback signal to reduce that distortion. Problematically, however, the degree to which such distortion can be reduced is limited by the degree to which that distortion can be observed in the feedback signal.